As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some lithium secondary batteries are commercially available.
A lithium secondary battery has a structure in which an electrode assembly, in which a porous separator is interposed between a cathode and an anode, each of which includes an active material coated on an electrode current collector, is impregnated with a lithium salt-containing non-aqueous electrolyte.
Electrode assemblies are classified into a jelly-roll type electrode assembly fabricated by interposing a separator between a cathode and an anode, each of which includes an electrode active material coated on opposite surfaces of a foil of a long sheet type as a current collector and winding the resulting structure and a stack-type electrode assembly fabricated by sequentially stacking a plurality of cathodes and anodes, each of which includes an electrode active material coated on opposite surfaces of a foil having a certain unit size as a current collector, with separators disposed therebetween.
In a stack-type electrode assembly, cathodes and anodes are alternately stacked. In this regard, it is difficult to accurately align cathode surfaces and anode surfaces with respect to each other and thus a lithium movement rate decreases and areas participating in reaction are reduced by areas of cathode and anodes that do not align with each other, which results in reduction in cell capacity.
Carbon-based materials are mainly used as anode active materials, and lithium cobalt-based oxides, lithium manganese-based oxides, lithium nickel-based oxides, lithium composite oxides, and the like are mainly used as cathode active materials.
Among such cathode active materials, as a representative example, LiCoO2 exhibits good electrical conductivity, high output voltage, and excellent electrode characteristics and is commercially available. However, LiCoO2 is disadvantageous in terms of economic efficiency according to natural abundance and raw material costs and in terms of environment, e.g., harm to human bodies. LiNiO2 is relatively inexpensive and exhibits high discharge capacity, but is difficult to synthesize and has thermal stability problems in a charged state. In addition, manganese-based electrode materials such as LiMn2O4, LiMnO2, and the like are easy to synthesize and inexpensive, exhibit good electrochemical discharge properties, and are less harmful to the environment and thus are widely applied as active materials. However, these manganese-based electrode materials have low conductivity and theoretical capacity and high operating voltage and thus an electrolyte is likely to decompose.
In addition, when a high-voltage cathode is used, an electrolyte is oxidized because it reaches an oxidation potential and thus gases and byproducts are generated and, accordingly, battery performance is deteriorated and resistance increases. Consequently, batteries have severe safety problems.
Therefore, there is an urgent need to develop a secondary battery that does not have such problems and operates at high voltage.